Modeling interfaces in polymer nanodielectrics

Thumbnail Image
Shandilya, Abhishek
Issue Date
Electronic thesis
Materials engineering
Research Projects
Organizational Units
Journal Issue
Alternative Title
Ab initio design of polymer nanocomposite materials for high breakdown strength requires prediction of trap states at the polymer–filler interface. Systematic first-principles calculations of realistic interfaces can be challenging, particularly for amorphous polymers and fillers that necessitate the calculation of ensembles of large unit cells with hundreds of atoms. We present a computational approach for automatically generating reasonable structures for amorphous polymer–filler interfaces, combining classical molecular dynamics and Monte Carlo simulations. We identify trap states by analyzing the localization of electronic eigenstates calculated using density functional theory on ensembles of interface structures, clearly distinguishing shallow trap states from delocalized band-edge states. Nanofillers in polymer nanocomposites are functionalized to improve dielectric performance in both direct and indirect ways. For comprehensive design of polymer nanodielectrics, we include coupling agents with functional groups to our automated scheme of generating interface structure. In our initial study, we create ensembles of interfaces with three functional groups - thiophene, terthiophene, ferrocene. Analyzing their eigenstates reveals distinct distribution of hole and electron traps in energy and space dimension dictated by the chemistry of functional groups. Apart from interface engineering, dispersion of nanofillers is an important factor in determining dielectric breakdown strength. Since simulating dielectric breakdown is challenging, we calculate electron mobility and calibrate experimentally measured dielectric breakdown strength. We use trap state information from either experiments to develop a Monte Carlo electron hopping model to simulate electron trajectories through a microstructure under a given electric field. We find a logarithmic relation between electron mobility and dielectric breakdown strength. A combination of ab initio, classical molecular dynamics and Monte Carlo methods applied in investigating amorphous interfaces of polymer nanodielectrics can be extended to other areas too. Understanding electrochemical interfaces is an equally-complex problem.Controlling electrochemical reactivity requires a detailed understanding of the charging behavior and thermodynamics of the electrochemical interface. Experiments can independently probe the overall charge response of the electrochemical double layer by capacitance measurements, and the thermodynamics of the inner layer with potential of maximum entropy (PME) measurements. Relating these properties by computational modeling of the electrochemical interface has so far been challenging due to the low accuracy of classical molecular dynamics (MD) for capacitance and the limited time and length scales of \emph{ab initio} MD (AIMD). Here, we combine large ensembles of long-time-scale classical MD simulations with charge response from electronic DFT to predict the potential-dependent capacitance of a family of ideal aqueous electrochemical interfaces with different peak capacitances. We calculate two charge-based benchmarks which indicate an asymmetric response of interfacial water that is stronger for negatively charged electrodes, while the difference between CME and CMC illustrates the richness in behavior of even the ideal electrochemical interface.
December 2021
School of Engineering
Full Citation
Rensselaer Polytechnic Institute, Troy, NY
PubMed ID